Signal transduction is a fundamental process that enables cells to acquire information from the extracellular environment and respond to this information appropriately. Molecular defects in this process are now recognized as a leading cause of a host of human diseases. Receptor tyrosine kinases (RTKs) are key mediators of signal transmission in response to extracellular cues that regulate cell proliferation and differentiation. It is widely accepted that differences in the duration and intensity of RTK-dependent signals constitute important determinants in dictating the nature of the biological response they elicit. However, relatively little is known about the regulatory principles that govern the quantitative output of signals generated in response to RTK activation. This application is motivated by the need to elucidate these principles with precision and to understand how they contribute to the signaling repertoire of RTKs at the cellular level. To accomplish this objective, the proposed studies will focus on the positive and negative modulation of the EGFR-Ras signaling axis by Son of sevenless (Sos) and Spry (Spry), respectively. The experimental plans consist of two broad aims: (1) To define the molecular framework for Sos-dependent coupling of EGFR to Ras activation. (2) To determine the functional interface between Spry and EGFR signaling. These investigations should advance our understanding of fundamental mechanisms that control the response capabilities of RTK signaling pathways and provide insights into molecular determinants that contribute to human disease.